Various mechanisms exist to control or limit turbocharger noise. For example, a turbocharger may be fitted with an acoustic damping mechanism (e.g., a noise shield, a muffler, etc.) that aims to damp noise. Other types of mechanisms aim to control or limit a source or sources of noise generation, which may also improve turbocharger performance. For example, where noise arises due to instability of a rotor-bearing system (RBS), a mechanism that enhances stability of the RBS may also reduce noise generated by the RBS. As described herein, the term “instability” is associated with rotordynamics (e.g., fluid dynamic pressure around a circumference of a rotor component) while unbalance is associated with components and assembled components (e.g., an unbalanced compressor wheel, shaft or turbine wheel).
Some types of turbocharger noise are associated with synchronous vibration (SV) while other types are associated with non-synchronous vibration (NSV), which includes sub-synchronous vibration and supersynchronous vibration. Synchronous refers to synchrony with a component such as a rotating shaft. For example, vibration that increases in frequency directly with shaft rotational frequency is considered synchronous (e.g., due to assembly unbalance). In contrast, vibration associated with a lubricant film is typically non-synchronous, i.e., while related to turbocharger operation, the relationship between shaft rotational frequency and vibration are not direct. For example, NSV may arise from an unstable RBS mode characterized by clearances, lubricant properties and particular operational conditions that occur from time to time. Such modes are often bound and may reach a limit cycle that limits amplitude. In other instances, NSV may be unbound and increase in amplitude, which can result in excessive vibration, noise and even destruction of components. When a turbocharger is coupled to an internal combustion engine, engine vibrations can also add to NSV, for example, engines are known to introduce significant and complex, low frequency subsynchronous whirl in turbochargers, which may be multiples of engine speed.
NSV can be the result of many design parameters. Control of these parameters is not always easy and, for some turbochargers, NSV may be unavoidable due to basic design requirements. Some mechanisms that aim to control NSV rely on specialized, modified or optimized rotor supports, which can have drawbacks such as increased cost, complexity or reduced component tolerance. To date, the associated drawbacks of such mechanisms seldom outweigh marginal reductions in NSV.
Of course, proper balancing of turbocharger components, individually and in various stages of assembly, can reduce noise generation. However, as noted, various non-synchronous vibrations are indirectly related to shaft rotational speed and therefore not addressed by balancing. For example, non-synchronous whirl may be caused by lubricant film dynamics, considered “self-excited”, that become sustaining at a certain shaft speed (e.g., “oil whip”). When components are assembled, so-called “stackup” imbalance may occur. For smaller turbochargers, assembly balancing can significantly reduce stackup unbalance, which is known to generate screaming or whining noise.
Balancing may be performed at “low” speed or “high” speed. Various commercially available balancing machines (e.g., “vibration sort rig” (VSR) machines) are configured for high speed balancing of turbocharger cores (e.g., cartridges, RBSs or center housing rotating assemblies (CHRAs)). A typical VSR machine-based balancing process supplies lubricant and drives a rotor using compressed air. Such balancing requires various manual steps, including noting heavy point(s) and manual cutting. Depending on the amount of unbalance, a person may need to repeat such steps, which adds cost. Various low speed balancing machines allow for two-plane balancing and can achieve acceptable results; noting that many high speed balancing machines only allow for single plane balancing. In either instance, i.e., low or high speed, balancing is a necessary cost for proper operation, noise reduction and longevity of turbochargers.
As described herein, a need exists for cost-effective mechanisms that can reduce turbocharger noise. Such mechanisms should not introduce complicated manufacture specifications or introduce additional component and assembly unbalance issues.